Análise das petrotramas das rochas Charnockiticas da Serra da Prata, Mucajaí / RR

Abstract

The Mucajaí Gneiss, Serra da Prata Charnokites, San Antonio and Pratinha Gabbros units of exhibit Sn gneissic and S1 mylonitic foliation, with deformation microstructures generated under medium to high amphibolite facies temperature conditions. Despite the obliteration of primary structures, some igneous microstructures preserved in granitoids and gabbros, such as preferred orientation of feldspar phenocrysts with crystal faces due to magmatic flow, submagmatic microfractures and dike intrusion controlled by pre-existing anisotropy (Pre-S1), suggest syn-magmatic deformation processes. U-Pb SHRIMP analysis of oscillatory zones of igneous zircons provided Concordia ages of 1916.7+4.5 Ma for a mylonitized granite of Serra da Prata Charnokites, 1905+5.2 Ma for associated gabbros and 1959+5 for orthognesis of Mucajaí Gneiss. These ages are interpreted as the crystallization ages of the protoliths and understood as the age of the tectono-magmatic event that probably controlled the emplacement of most of the granitic rocks of Serra da Prata Suite and associated gabbros. Another U-Pb SHRIMP age of 1863+6.7 Ma recorded in dynamically recrystallized zones of zircons from mylonitized granite is related to the metamorphic-deformational event that generated the foliation S1 and probably related to the generation of the Sn foliation. Reworking of deformational structures is interpreted by the presence of mylonitic recrystallized bands oriented parallel to S1 and oblique to Sn foliations. Temperature conditions for this deformation event, characterized by grain size reduction of the of the crystals, is assumed equivalent to upper green schists to medium amphibolite facies and interpreted as tectonic reactivation at shallow crustal levels under a regional stress field similar to that responsible for the registration of the foliation S1 and related to K’Mudku event. Magnetic mineralogy studies indicate that the principal magnetic marker of magnetic subfabrics in granitoids and gabbros is magnetite multidomain. At Sierra da Prata Charnokites magnetic sub-fabrics display a magnetic foliation oriented preferentially NE-SW. On the other hand, the Pratinha Gabbros display a random orientation of the magnetic foliation that is related to the heterogeneous distribution of deformation, concentrated along the margins with high dip angles and high values of magnetic anisotropy Pj (high strain). Less intense deformation was found at the center of the gabbroic body that displays sub-horizontal magnetic foliation with low values of magnetic anisotropy Pj (low strain). Given the nature of the internal deformation and the arrangement of the magnetic sub-fabrics, it is interpreted that S1 and Sn rock fabrics were produced by deformation under medium to high amphibolite facies temperature conditions related to dextral transpressional regime induced by NW-SE shortening active during the Paleoproterozoic.